Many MEMS include a released structure which has a high aspect ratio (AR) member in which a longitudinal length of the member is at least five times larger than a transverse length of the member or a member spaced apart from another structure by a gap defining a space with a high AR. High AR members and/or associated gaps are useful for providing large capacitances. In the case of an accelerometer, a high capacitance structure translates into greater device sensitivity. In the case of an electrostatic motor, a high capacitance enables a high electrostatic force between the released structure and a surrounding drive electrode. A high force allows for released structure to be actuated over a large distance or angle at a lower applied voltage, for improved electrostatic motor performance. Even for MEMS implementations which do not need a large actuation angle, a high electrostatic force allows flexures to be mechanically stiffer to increase the resonant frequency of the released structure and overall reliability of the device in an operating environment.
Another consideration in many MEMS is fill factor, which for a micromirror array implementation is a ratio of the active refracting area to the total contiguous area occupied by the lens array. To maximize the fill factor, it is beneficial to suspend the high aspect ratio member with the longest dimension oriented perpendicularly to a surface of the mirror, as is described for actuator members in commonly assigned U.S. Pat. No. 6,753,638.
Whatever the MEMS application and however a member may be oriented, it is challenging to ensure the center of mass of the released structure is best positioned with respect to the structure's center of rotation (i.e., fulcrum). For example, where a released structure has uniform density, and the center of mass is the same as the centroid of the structure's shape, a high aspect ratio member will often cause the centroid to be offset from a plane containing the released structure's center of rotation.
FIG. 1 is a perspective view illustrating a schematic of a released structure 100. The member 111 has a high AR, such as a beam or a blade, with a longitudinal length 150 that is significantly longer than at least one of the transverse lengths 140 and 130. The high AR member 111 is coupled to a substrate (not depicted) via a flexure which is the center of rotation for the released structure 100 within the X-Y plane (192, 191) only while other flexures may further allow motion within the Z-direction (into/out of the page) 193. Because the high AR member 111 has a finite mass, the center of mass (CM) is located within the high AR member 111, a distance 138 below the center of rotation 135 (e.g., 50-100.mu.m). With a center of mass offset from the center of rotation, the released structure generally forms a pendulum subject to motion in response to external forces (e.g., vibration). Such motion is undesirable as it corresponds to noise during operation of the MEMS.